hi

i'm just getting round to my speedo, at what speed do they test the readings are correct at the iva. this is were i need some help/advice i have got an engine out of a lotus elise and the lotus dash and loom. i have got the speedo reading a speed dont know if it is correct or not, i just picked up the back end and started the car in gear and got a reading on tick over and it moved when i revved the engine. the lotus has the speedo sensor picking up on the rear wheel/driveshaft so i have done the same i have mount a trigger wheel on my diff bolts, the lotus has a trigger wheel with 42 points and is 80mm in diameter. i got a trigger wheel made with 42 points but it is 120mm in diameter.

so how do speedo sensors work do they just count the number off points (42) as 1 full revolution and dia of the trigger wheel does not make any differance????? or will the size make a differance????

and i dont know what ratio diff the lotus used but i am using a sierra 1 which could also make the reading different, am i thinking on the right lines or just talking poo.

the size of the rear wheels will also make it different???????

well so what i thought if i took the car out for a spin, to see if the speedo was out, using my sat nav or somebody driving in front of me at a set mph and see what mine reads compared to theres. if its out which with all the varibles i have it will be could i alter the amount of pick up points on the trigger wheel. i've got 42points which is 1 complete turn of the driveshaft. so if the speedo was running slow put less pick up points on the trigger wheel, or more if its running fast.

how does it sound advice would be great thanks steve

ps i have got a price for a speedo healer which is £130 so i thought this would be a locst way.

They test to 70mph on the rollers
The diff ratio and tyre size will make a difference and will not read correct unless it is the same as the Elise

if the trigger wheel is on the driveshaft /diff output flange,
a) the diff ratio will make no difference,
but
b) the overall wheel and tyre size will.